Reheat furnaces are used to heat workpieces prior to being introduced into subsequent processing such as rolling into sheet material. The uniformity of such heating of the workpieces has a substantial effect on the end product (i.e., the sheet material) as well as influencing the reheat furnace thermal efficiency. Such metallurgical furnaces include walking beam types. In the walking beam type of furnace, multiple support rails extend for the length of the furnace and support the workpieces as they are being conveyed through the furnace by the moving (walking) beams.
Three separate heat transfer effects of this walking beam type furnace and support rail configuration limit the heating efficiency and temperature uniformity of the heated workpieces. The reduced heating effects are characterized as "cold spots" or localized cooler regions within the heated workpiece having distinctly different temperatures with locations corresponding to the respective support rail positions. The first heat transfer effect is due to the close proximity of the support rails to the workpiece. This proximity produces a shadow effect whereby the support rails block radiation or shade the workpiece from the heat source of the furnace. A second heat transfer effect is that rails tend to cool the workpieces by conduction in response to direct contact of the workpiece with the skid buttons or support elements affixed to the top of the support rail. The support rail is typically liquid cooled and has a temperature which is cooler than that of the skid buttons or support elements. The third heat transfer effect is that the rails tend to cool the workpieces by secondary radiation heat exchange occurring between the cooler top surface of the liquid cooled rail interacting with the bottom of the workpiece.
Efforts have been made in the past to increase heating uniformity and to minimize cold spots by adding insulation to the support rails. U.S. Pat. Nos. 4,095,937 and 4,228,826 show such support rails. Support rail insulation is widely employed within the industry to counteract the localized workpiece cooling by secondary radiation exchange with the liquid cooled support rails. Other attempts to solve this problem are disclosed in U.S. Pat. Nos. 4,427,187 and 4,368,038 which involve various "hot rider tiles", "skid buttons", or support elements which are attached or otherwise mounted directly to the support rails. These prior inventions represent the current state of the art and employ high temperature resistant (ceramic and/or alloy) materials to provide a suitable high temperature wear surface or support element with sufficient thickness and height to elevate the workpiece above the support rails and reduce the shadow effect.
The use of the state of the art walking beam reheat furnace support rails and support elements (skid buttons) persists as previously described, and continues to suffer from reduced heating efficiency and poor temperature uniformity. The history and use of these conventional designs supports particular notions concerning furnace heat transfer effects. Skid system engineers and designers have long understood the radiation viewfactor benefits and have attempted to achieve practical elevated skid button designs so as to distance the workpiece from the necessary, but detrimental, underlying liquid cooled support rails. Using such conventional design guidelines, high temperature alloy skid button materials are used which have demonstrated practical height limits of three to four inches (75-100 mm) when operated continuously within a 2,400.degree. F. (1315.degree. C.) reheat furnace. Above the practical height limit these components soon fail in service due to heat-checking, sigma phase embrittlement, plastic deformation and general overheating as the temperature dependent physical material strength limits are repeatedly reached and exceeded by actual furnace operation and service. To achieve practical and durable skid buttons, U.S. Pat. No. 4,293,299 addressed these physical material limits with direct additional skid button cooling by submerging the root or bottom of the support button within the liquid cooled stream of the main support rail. This development advocated the additional skid button cooling to reduce material operating temperatures. However, the intent of this patent was only to enhance the durability and longevity of the skid button. Normally, skid buttons are operated continuously in the harsh reheat furnace environment. The prior art has noticably ignored the separate direct cooling of an elevated skid button.
U.S. Pat. No. 4,609,347 proposed high temperature refractory skid button materials. While still under development, ceramic and admixture-type skid buttons of this height generally fail prematurely due to the inherently low compressive and mechanical strength of ceramics when submitted to a typical industrial reheat furnace service.
The foregoing is offered to confirm the previous reluctance of furnace skid system designers to significantly elevate workpieces above the support rails. The direct cooling of elevated skid buttons has previously been ignored because of the perceived detrimental effect caused by localized workpiece conduction cooling. If the skid buttons were cooled, it was typically reasoned that any product placed on top of these cooled skid buttons would also be cooled by conduction. This was considered to be against the desired purpose of the reheat furnace. As such, no attempts have been made, in the past, to directly cool elevated skid buttons. Typical skid buttons are of a height much less than four inches and are made of solid material. It has been the typical goal, in the prior art, to have the skid buttons retain as much heat as possible.
Through exhaustive heat transfer calculations of the heating process, the present inventor has identified, quantified and evaluated the pertinent design criteria governing the furnace heating efficiency and final workpiece temperature uniformity. It was found that elevating skid buttons significantly separate or distance the workpiece from the support rails and thereby greatly enhance workpiece heating. Additional beneficial radiation heating from the hot furnace (the first heat transfer effect identified hereinabove), and re-radiation from the hotter rails (previously identified as the third heat transfer effect) have been found to overcome, and indeed, far outweigh the detrimental conduction cooling effect (identified as the second heat transfer effect hereinabove) incurred by direct workpiece contact with the individually cooled elevated skid buttons.
It is an object of the present invention to provide increased heating efficiency and workpiece temperature uniformity as produced by walking beam type reheat furnaces.
It is another object of this invention to suitably and practically support workpieces a significant distance above the skidrails or support rails to minimize detrimental shadow effects and increase radiation heating from the hot furnace to the workpiece.
It is another object of the present invention to elevate workpieces a significant distance above the support rails to allow heating of these rails and to promote additional workpiece heating by re-radiation at these locations.
It is another object of the present invention to effectively support workpieces a significant distance above the support rails by means of individually cooled extensions from the support rails.
It is still a further object of the present invention to reduce the conduction effects by skewing the workpiece support along the support rails.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.